This is part of one the Society's forms used from 1992. The paragraph that says, "If the individual is reinstated (or dies), the secretary should fill in the date and send it to the Society in the special blue envelope" caught me off guard. OR DIES?? Did they really keep track of when former members died and sent it off to the Society in a "special blue envelope"? That just seems so weird ... and well ... culty. Do they still do this?
What kind of data does the Watchtower keep on former members?
If by "former members" you mean DF'ed or DA'ed, then they keep whatever is on the "S-77" form.
In years past, there was a question on the form in which the elders had to explain what the person was disfellowshipped for. There was room for about a paragraph, but elders could send in an attachment if that wasn't enough space.
On the current S-77, the question asks merely for the "scriptural reason" for the DFing. There is room for a one or two word answer ("porneia", "drunkenness", "smoking", etc.). No details are to be included.
The WTS keeps the S-77 until they receive notice that the person has been reinstated or has died. Maybe they keep it even after reinstatement, who knows.
I imagine, though am not sure, that when the new S-77 form came out (a few years ago), the one without details, that they transferred info from the old forms onto the new ones (name, date, etc.) and then destroyed the original, detailed ones.
TL;DR: They keep whatever is on the official DF form (S-77). The official form provides much less detail now than it did years ago.